106 JOURNAL OF SCIENCE. 
lar to that existing between small-pox and the slight illness 
produced by vaccination. This experiment was followed up, 
and it was found that cattle which had had the mild form of 
the disease by inoculation from a guinea-pig were thereafter 
safe from infection with the ordinary or fatal form. They had 
in fact been “ vaccinated” ‘to extend the meaning of that word) 
for splenic fever. 
M. Pasteur tried some experiments with a view of actually 
curing animals suffering from Distemper. It was known that 
birds were not subject to this disease. Pasteur thought that, as 
Bacteria are often very susceptible to temperature, this circum- 
stance might be due to the heat of their blood, which is about 
10° or 12° higher than that of mammals. He artificially lowered 
the temperature 9f birds, by keeping their feet in cold water, and 
then inoculating them produced a fatal attack of the fever. 
Conversely he raised the temperature of inoculated guinea-pigs 
—otherwise doomed to certain death—and saved their lives. 
These researches point to the possibility of preventing or 
curing the terrible infectious diseases which make such havoc 
among us. It may be urged that life will be hardly worth having 
if we are to be “ vaccinated,” not only for small-pox but for ty- 
phus, diphtheria, whooping-cough, measles, consumption, and 
the rest ; but the fact remains that the outworks of one of the 
most terrible enemies of the human race have been successfully 
stormed: may we not hope that, sooner or later, the capture of 
the citadel is certain ? 
I may point out that the practical importance of the germ 
theory is reduced to a minimum if the theory of spontaneous 
generation be true, since in that case Bacteria may at any time 
be formed spontaneously in the tissues, and no efforts to keep 
out germs will be of any avail. 
I have taken splenic fever as a type of infectious diseases, be- 
cause it is the one which has been most thoroughly worked out 
and in which the fact that Bacteria are the actual cause of the 
symptoms is most certain ; but other complaints are being inves- 
tigated, and already the bacterial nature of several is no longer 
doubtful. 
One case in which I believe there can be no longer any 
doubt is that of tuberculosis, the lung disease which is the chief 
symptom of consumption. As in the case of splenic fever, Koch 
has discovereda bacillus whichis invariably present in tubercleand 
in the saliva of consumptive patients, and which, cultivated apart 
from the living body, is capable of producing the disease by 
inoculation. 
Similarly, erysipelas is certainly, and diphtheria and pneumo- 
nia are probably, due to species of Micrococcus, a small globular 
form of bacteria ; chicken-cholera, a fatal disease from which the 
domestic fowl suffers, to a true Bacterium ; glanders to a Bacillus ; 
and relapsing fever to a Spirochcete, a sort of very long and 
flexible Spirillum. There is also some reason to think that many 
other diseases, such as scarlatina, measles, typhus, dysentery, 
